But Margaret McCartney, a Glasgow GP, said there was no firm evidence backing up claims that drinking so much was necessary to prevent dehydration.
She said the recommendation was "not only nonsense, but thoroughly debunked nonsense".
Writing an opinion piece in the BMJ, she said several studies showed no clear benefit of drinking large amounts of water.
For example, she said that in 2002 Prof Heinz Valtin from Dartmouth Medical School in the US, published a review of the evidence in the American Journal of Physiology, concluding there was "no scientific evidence that we need to drink that much".
Unsurprisingly, bottled water companies were keen to push the advice as a public health message.
Danone, for example, the French company behind the Volvic and Evian brands, recommends drinking up to two litres (eight standard glasses) daily as "the simplest and healthiest hydration advice you can give."
It also claims that "even mild dehydration plays a role in the development of various diseases."
However, Dr McCartney wrote that there was no high quality published evidence to support these claim.
David Graham, director of Danone's Hydration for Health initiative said: "Our view on how much water you should drink is in line with the European Food Safety Authority's 2010 scientific opinion on water intake.
"The amount a person should drink is dependent on personal factors, such as sex and age, and EFSA's opinion to drink 1.5l to two litres of water per day is sensible for a normal woman or man."
